Some radiation detection elements for detecting radiation such as X-rays use a semiconductor. An example of the radiation detection element using a semiconductor includes, a silicon drift detector (SDD). Around the surface of a radiation detection element using silicon (Si), an interface between a Si layer and SiO2 which is an insulating film is present. In a depleted region in the interface, an electric charge not derived from radiation is generated. The electric charge not derived from radiation is an electric charge different from the electric charge generated by the entrance of incident radiation. The electric charge generated in the depleted region in the interface is to be leak current, causing noise.
Positive electric charge fixed in the insulating film generates at the interface an accumulation layer in which negative charges may reside. In the case where the accumulation layer is present, the generation of electric charges is suppressed. However, at operation of a radiation detection element, voltage is applied to the radiation detection element, which generates an electric field around the surface of the radiation detection element. The electric field affects the accumulation layer, which forms a depleted region at the interface, and makes it easy for an electric charge to be generated. Japanese Patent Application Laid-Open Publication No. 2014-2155 discloses a technique of collecting electric charges generated around the surface of a SDD for reducing noise.